1. Field of the Invention
The present invention relates to improvements in the use of Raman amplification in distributed optical fiber sensing systems, such as optical time domain resolution systems.
2. Description of Related Art
There is an interest in extending the range of distributed optical fiber sensors, in which backscattered light produced by a propagating probe pulse is collected to yield measurements of parameters associated with the fiber. Of particular interest are systems based on Brillouin and/or Rayleigh backscatter. A recent system [1] proposed a combination of distributed Raman amplification over the first 50 km of a sensing fiber and discrete erbium doped fiber amplifiers at 50 and 75 km, and was demonstrated to achieve 100 km range in the case of the determination of the Brillouin frequency shift with a resolution of well below 1 MHz (equivalent to better than 1 K resolution). To achieve this, continuous-wave pump power (460 mW) was co-propagated with the probe pulse, thus resulting in Raman gain for the probe pulse and also for any backscatter power falling within the gain bandwidth of the Raman process. However, it was found necessary to reduce the probe pulse power in order to prevent the undesirable non-linear effects that occur when a probe pulse exceeds a certain power level. As a result, the point within the system where the performance was worst was nearest to the acquisition equipment. Clearly, the gains could be balanced more effectively, but a fundamental problem with such a system is that the gain for the probe pulse cannot be adjusted independently of that for the backscatter. In addition, the gain experienced by the probe pulse depletes the pump power locally, which means that the probe pulse is amplified when it is strongest, and then when its intensity has decayed there is no more pump power available to amplify it.
Earlier work using Raman amplification [2, 3] did not co-propagate the pump power from the interrogation equipment so the problems discussed above did not arise. An alternative system uses remotely-pumped fiber amplifiers [4]. Attempts to control the Raman gain have been made [5, 6], using pulsed Raman amplification such that Raman gain for the probe pulse occurs only some distance along the fiber. By timing the launch of a pump pulse and a probe pulse, the position at which they overlap can be controlled owing to the difference in their propagation velocities. In this way, the probe pulse can be allowed to decay somewhat before being amplified by transfer of energy from the pump pulse, to limit nonlinear effects. A drawback of this approach, however, is that the probe pulse is not amplified uniformly since one of its edges overlaps with the pump pulse before the remainder of the probe pulse and thus this edge receives all the amplification. As a result, distortion of the probe pulse may result which in turn might cause non-linear effects, notably self-phase modulation. In addition, this approach does not allow the returning backscattered light to be amplified at the same time, because the probe power is confined to the duration of the probe pulse.